We often hear about terms like Typhoon, Hurricane, Cyclone, Tropical Depression and Tropical Storm.  So what do they all mean?  And what are the processes to actually create a hurricane?  

With the media-grabbing Atlantic Hurricane season upon us we thought we’d take a closer look.

Firstly whether you call it a Typhoon a Hurricane or a Cyclone they are all the same thing – they just form in different parts of the world. 

Three Names for the Same Thing

• Tropical Cyclones – form in the Southern Hemisphere (cyclonic)
• Hurricanes – form around America – either in the Atlantic Ocean or in the eastern Pacific Ocean (east of the International Dateline)
• Typhoons form in the western Pacific (west of the International Dateline).  This means a Hurricane that moves through the International Dateline would then become a Typhoon.

Another thing all these tropical storms have in common is how they are made…

How a storm is made

• Step 1:  A tropical ‘wave’ is needed.  This is literally a wave in the air, or a disturbance or the normal easterly flow.  The wind tries to move another direction and it causes the air pressure to start falling – and the air to start spinning – creating a low.
   
• Step 2:  This then forms into a tropical depression.   It’s given a number.  For example, TD2 (for the second Tropical Depression of the official storm season).
   
• Step 3:  Wind speeds start to average gale force – officially making it a Tropical Storm.  It’s at this point the storm is named.  In our part of the world this would become a Category 1 cyclone.
   
• Step 4:  There are some more technical things that need to go on, but once the winds increase we start stepping up the categories.  Different regions use different ways to measure a storm’s strength which is a bit of a pain but considering we’re focusing on the American Hurricane Season we’ll use there system – the Saffir-Simpson scale.  

Hurricane Categories

• A Category 1 hurricane starts with winds averaging 119km/h. (In NZ we call that severe gale).  
   
• Category 2 has sustained winds of 154km/h (we very rarely see anything like that in NZ – usually only gusts.)
   
• Category 3 has sustained winds of 178km/h
  
• Category 4 has sustained winds of 210km/h.
   
• Category 5 has sustained winds of 250km/h (and the air pressure is below 920hPa).